Tungsten hexafluoride is the source gas used in the chemical vapor deposition of tungsten for microelectronics manufacturing process. In these applications, UHP tungsten hexafluoride is needed to assure reliability of the semiconductor manufacturing processes.
Various attempts have been made in the manufacture of UHP tungsten hexafluoride. Tungsten hexafluoride may be produced by the reaction of high purity tungsten metal with fluorine. However, there are difficulties in controlling the impurities in the fluorine source which often results in high level of gaseous impurities in the 100 to 1000 ppm range. The reaction between fluorine gas with trace moisture in the wall of the gas handling system which results from the manufacturing process may produce additional hydrogen fluoride impurities. Moreover, the presence of impurities in the source tungsten metal may result in the incorporation of low volatile metallic impurities in the final product. To achieve a UHP grade tungsten hexafluoride product, a more reliable and economical purification process is needed. More specifically, it is desired to devise a comparatively simpler method for producing a UHP grade tungsten hexafluoride of 99.9999% purity from a crude grade tungsten hexafluoride of about 99% purity than presently available in the art.
Certain attempts have been made to purify tungsten hexafluoride. U.S. Pat. No. 5,324,498 discloses a purification process for tungsten hexafluoride which involves a) evaporation of tungsten hexafluoride to remove non-volatile resides, b) condensing the evaporated product, c) freezing the condensed tungsten hexafluoride to solid form, d) evacuating the headspace to remove volatile impurities, e) thawing the solid and heating the tungsten hexafluoride to a temperature above its boiling point at a reduced pressure, f) evacuating the headspace to remove volatile impurities from the headspace, and g) repeating the thermal cycles in steps c to f at least once. The process in the '498 patent has a marginal success for removing volatile impurities such as nitrogen and oxygen, and is not effective for removing hydrogen fluoride in the tungsten hexafluoride due to the similarity in the boiling point temperature. Since the process requires several repeated thermal cycles, the process is very time consuming.
U.S. Pat. Nos. 5,328,668 and 5,348,723 disclose synthesis processes for producing semiconductor grade tungsten hexafluoride. These processes use evaporation as the means to remove non-volatile impurities from the product and does not provide any means for removing other metallic impurities as well as a methods for reducing the hydrogen fluoride level.
U.S. Pat. No. 3,995,011 discloses the production of tungsten hexafluoride from tungsten, halogen and hydrogen fluoride. However, there is no teaching or suggestion for the purification of tungsten hexafluoride.